Many applications exist in present day electronic equipment for bipolar amplifier circuits having high input impedances. More specifically, it is desirable to use such amplifiers with high impedance, ceramic transducers to provide maximum power transfer between the transducer and the amplifier and to satisfy frequency response specifications. Unfortunately, bipolar transistors and their associated biasing networks do not normally provide a high enough input impedance for such applications.
Positive feedback techniques have been utilized in bipolar amplifiers to increase the apparent input impedance thereof by a technique referred to in the art as "boot-strapping". Circuits utilizing discrete components normally include bootstrap capacitors for applying positive feedback from the amplifier output terminal, to the amplifier input terminal. These capacitors tend to increase the cost of the prior art discrete circuits. Furthermore, such circuit configurations are not suitable for fabrication in monolithic form. This is because the required amount of capacitance of the bootstrap feedback capacitor is greater than the amount of capacitance which can be readily and inexpensively provided on a monolithic integrated circuit chip. As a result, if the discrete circuit configuration was fabricated in monolithic form, the bootstrap capacitor would have to be provided external to the monolithic chip. Hence, leads or pins would have to be brought from the chip to connect to the capacitor which would undesirably increase the size of the integrated circuit and prevent these pins from being available for other functions.
Accordingly, prior art circuits have been developed which eliminate the need for the bootstrap capacitor. These circuits however suffer from other shortcomings when provided in integrated circuit form. More specifically, these other prior art circuits sometimes have quiescent operating voltages which depend on the somewhat unpredictable betas of PNP transistors, the degree of matching between current sources and have large undesired temperature caused variations. Also these prior art circuits often require large frequency compensation capacitors for preventing internal oscillation. As a result, the quiescent operating points of individual circuits tend to vary even at constant temperatures between different batches of wafers because of normally uncontrolled processing parameters. Because of signal drifts with temperature and variations with processing parameters, it is difficult to rapidly test the resulting amplifier circuits which increases their cost. Moreover, it is difficult to design circuits that can be direct current (DC) coupled to the output terminals of such amplifiers because the quiescent output voltage of such amplifiers are not constant. Also, such prior art circuits are often unnecessarily complex.